Metal filter media are used in a wide variety of applications due to their excellent structural integrity, versatility and ability to withstand high temperatures. In high temperature applications, metal media offer a significant advantage over ceramic filters due to the inherent brittleness of the latter. Most metals do not exhibit the same degree of brittleness, but rather show some degree of ductility, i.e., the metal can be stretched, drawn or hammered thin without breaking. Filter media formed from ductile metals are resilient and able to withstand shock or vibration.
Metal filter media can be produced in a wide variety of pore sizes and configurations and are typically formed by sintering fine metal particulates, such as metal particles or metal fibers. Fine metal fibers are particularly desirable, because metal fibers permit the formation of filter media which, despite having a very small pore size, have a low pressure drop (.DELTA.p). The production of such media may require the use of metal fibers with extremely small diameters, often as small as about 2 micrometers in diameter or within a range from about 1 to about 4 micrometers.
The type of application or lifetime of a metal filter medium, however, may be limited by the ability of the base metal to resist undesirable oxidation. For the purposes of this invention, undesirable oxidation is defined as a chemical reaction that alters the chemical and physical properties of a metal in a detrimental manner. Oxidation here has the meaning typically ascribed to it by a chemist, i.e., at least one metallic element in the alloy increases in its oxidation state or valence. Undesirable oxidation includes both corrosion and high temperature oxidation of a metal. A variety of conditions, including exposure to aqueous acid or halide-containing solutions, contact with corrosive gases and the combination of high temperatures with an oxidizing or sulfur-containing environment, can foster the development of undesirable oxidation reactions. Corrosion may occur in a variety of forms, including general corrosion, pitting and crevice corrosion (which occur particularly in high chloride environments), stress corrosion cracking, hydrogen embrittlement and intergranular corrosion. Hydrogen-assisted stress-cracking may develop when metal components are stressed in the presence of hydrogen or hydrogen sulfide. Undesirable oxidation may cause a buildup of oxidation products or the erosion of metal, thereby weakening the physical integrity of the filter medium. Ultimately, undesirable oxidation can result in failure of the medium due to cracking, channeling or clogging.
It is well known that the resistance to undesirable oxidation of a particular metal alloy may be improved by enriching the alloy with certain treatment metals, such as chromium, nickel or aluminum. The addition of chromium to a metal may provide increased resistance to general and pitting corrosion. Ferrous alloys containing varying amounts of aluminum show enhanced resistance to oxidation and sulfur attack at high temperatures. Nickel containing alloys are known to have good resistance to pitting and crevice corrosion and are often used in chloride containing environments. Titanium- or columbium-stabilized grades of stainless steel exhibit enhanced resistance to intergranular corrosion.
The sintering process used to form metal filter media places limitations on the metal alloys that may potentially be employed. Certain highly reactive elements, if present in even very minor amounts, will seriously interfere with the sintering process. For example, the presence of a few hundredths of a per cent of aluminum in an alloy will have an adverse effect on sintering and the presence of a few tenths of a percent of aluminum will make it difficult to sinter a material at all. Other elements, such as chromium, are especially prone to volatilize during vacuum sintering, thus making it difficult to maintain high concentrations of such elements. In addition, the high temperatures required for sintering will tend to result in the formation of filter media in which the alloy composition is substantially uniform throughout the profile of the sintered metal particulates. While increasing the amount of a specific element throughout the profile of the sintered metal particulates may enhance the resistance of the surface to undesirable oxidation, the physical properties of the core of the particulate may be adversely modified.
Unfortunately, for applications where enhanced resistance to undesirable oxidation is required, many alloys are simply not available as fine metal fibers. Difficulties with the drawing process (e.g., high work hardening rates, nonmetallic inclusions, carbides, precipitated phases, and refractory oxides) limit the diameter, to which a fiber of more heavily alloyed grades, can be drawn. For instance, Hastelloy X is not available in diameters finer than 8 micrometers. Similarly, Fecralloy is only commercially available with a minimum fiber diameter of 22 micrometers. Fine stainless steel fibers, having diameters of less than about 2 micrometers, are essentially only available in 316 L stainless steel.
The availability of metal filter media having a wider variety of higher alloy compositions would greatly increase the range of applications where such media could be employed. For some time, the electronics industry has had a need for fine filters capable of filtering the highly corrosive gases, such as chlorine, hydrogen chloride and tungsten hexafluoride, used in chip fabrication. The nuclear power industry requires fine filters able to withstand superheated water at temperatures of 300.degree. to 350.degree. C. Other industries have filter applications which involve exposure to high service temperatures (e.g., greater than 350.degree. C.) in the presence of combustion gases or oxidizing environments such as air. High temperature gaseous environments having an appreciable sulfur content present a particularly difficult corrosion problem.
It would be particularly desirable if metal filter media formed from very fine metal fibers, with their inherently low pressure drop, were available in a wider range of alloys. There is a continuing need, therefore, for porous metal media that can be used in environments which cause undesirable corrosion. In particular, there is a need for fine metal filter media and the fibers from which such media are formed, having improved resistance to corrosion and/or high temperature oxidation.